“Wasting disease” and other dynamic phenomena in <i>Zostera</i> beds

The greatest decline of seagrass beds that is known is certainly the almost simultaneous breakdown of the North Atlantic populations of Zostera marina L., due to the “wasting disease”. Several explanations have been presented for this ecological disaster; however, none of those is satisfactory. It appears that for several localities a local explanation can be given. Apart from this general decline, it appears that, at least in Western Europe, there is a great variation in the temporal and spatial development cycles of the intertidal Zostera-beds. These variations can be ascribed to a number of factors, such as winter temperature (frost or no frost), grazing by birds (grazing or no grazing) and the balance between sedimentation (including sanding) and erosion. The combination “frost-grazing” leads to an annual cycle of the biomass, and a homogeneous bed structure. The combination “no frost-no grazing” causes raising of the seagrass bed, due to sedimentation up to a level where the environmental conditions become less favourable to seagrass; then breakdown sets in. These beds are heterogeneous and show a pluriannual developmental cycle.